The present invention relates to display panels and, in particular, it concerns dynamically controlled front-lit display panels based on liquid crystal technology.
It is known to employ liquid crystal technology for display panels. Typically, such panels are of back-lit designs in which the display is generated by controlling the amount of active rear illumination reaching the eye of a viewer. Display panels of this type are difficult to view under bright ambient lighting conditions when levels of reflected light am typically equal to, or greater than, the active display illumination,
In an attempt to produce a display screen which may be used under bright ambient lighting conditions, front-lit reflective displays have been developed. An example of such a display is disclosed in U.S. Pat. No. 5,541,749 to Konuma et al. which is hereby incorporated by reference as if set out filly herein. Displays of the reflective type operate by modulating the state of a liquid crystal layer so as to vary its reflectivity. Thus, in the case of polymer-dispersed liquid crystal (PDLC), the layer can be varied from highly reflective milky-white dispersive state to a clear transmissive state. By placing colored or black layers behind a pixel, it is possible to vary the pixel from white to a colored or black appearance.
Some conventional PDLC technology suffers from an inability to produce dark colors or high contrast. Since each pixel switches between a white maximum reflectivity state and a colored minimum reflectivity state, the display produces a range of pastel colors which contain a large admixture of white. The available contrast can be somewhat enhanced by the provision of pixels with black backing, but the darkest effect available still includes a combination of colors reflected from all the colored pixels, producing an off-white effect.
Further problems are posed by the need for independent electrical control over each pixel of a reflective display screen. Thus, for example, the design of Konuma et al. resorts to a complicated arrangement of thin film transistors for switching the light-modulating layer. Such complicated designs are also typically unsuitable for large scale applications such as outdoor display panels for advertising and the like.
U.S. Pat. No. 5,384.067 to Doane et al. discloses a light-modulating device based on a polymer stabilized chiral nematic liquid crystal material. By applying suitable pulses of an electric field, the device can be adjusted to a stable state in the continuum between a substantially transparent state and a reflective state. The reflective state selectively reflects a given color the wavelength of which is fixed by the pitch length of the liquid crystal. The pitch length, in turn, is fixed by the proportions of nematic liquid crystal and chiral material which are used. The aforementioned patent is hereby incorporated by reference as if fully set out herein.
Advances have also recently been made in development of surface stabilized cholesteric liquid crystal which has similar optical properties to the polymer stabilized chiral material but has advantages in its production process. This technology is described in an article by Z. J. Lu et al. (1995 SID Proc., pp. 172-175) which is hereby incorporated by reference as if fully set out herein.
Practical instructions for producing both polymer stabilized and surface stabilized liquid crystal cells of different colors are given in a brochure entitled xe2x80x9cMerck Liquid Crystalsxe2x80x94Licrilite(copyright)xe2x80x9d (Version 4, Dec. 1995), available from Merck Ltd., England. The materials required are commercially available from the same source.
Other recent developments have achieved similar functionality using PDLC materials by introducing dichroic dyes into the liquid crystal droplets. Since the optical path of light through the PDLC layer is very much greater in the scattering state than in the clear state, the overall effect is of a colored scattering state while the transparent state remains almost uncolored.
There is therefore a need for a front-lit display panel which is simple in structure, easy to produce, provides high contrast and dark color capabilities, and is suitable for large scale applications.
The present invention is a front-lit display panel assembled from a plurality of independent pixel units.
According to the teachings of the present invention there is provided, a front-lit display panel comprising: (a) a plurality of pixels units, each pixel unit having: (i) a layered structure including a sequence of: a transparent conductor, an active layer containing liquid-crystal-type molecules, and a rear conductive layer, parts of the pixel unit which lie on a side of the rear conductive layer remote from the active layer being designated as the xe2x80x9crear portionxe2x80x9d of the pixel unit, (ii) a first electrical connector in electrical contact with the rear conductive layer and extending to the rear portion of the pixel unit, and (iii) a second electrical connector in electrical contact with the transparent conductor and extending to the rear portion of the pixel unit; and (b) an assembly board for receiving the plurality of pixel units at a plurality of pixel receiving positions arranged in a two-dimensional array, each pixel receiving position having at least two contacts adapted for making electrical contact with the first and second electrical connectors of one of the pixel units.
According to a further feature of the present invention, the active layer contains a colored dye.
According to a further feature of the present invention, there is provided a colored filter positioned in front of the transparent conductor.
According to a further feature of the present invention, the active layer includes either PDLC or chiral nematic liquid crystal material.
According to a further feature of the present invention, the assembly board has a plurality of sockets, one of the sockets being located at each of the pixel receiving positions, and wherein the rear portion of each pixel unit includes a projection shaped for engaging one of the sockets.
According to a further feature of the present invention, the first and second connectors include connecting strips associated with at least one external surface of the protection.
According to a further feature of the present invention, the rear conductive layer is transparent, and wherein the rear portion of the pixel unit includes black backing material.
According to a further feature of the present invention, the plurality of pixel units includes a first group of pixel units in which the active layer assumes a relatively high reflectivity state in which a first color is reflected, a second group of pixel units in which the active layer assumes a relatively high reflectivity state in which a second color is reflected, and a third group of pixel units in which the active layer assumes a relatively high reflectivity state in which a third color is reflected.
According to a further feature of the present invention, one of the transparent conductor and the rear conductive layer is subdivided into a plurality of discontinuous regions, each of the regions being provided with a separate electrical connector in electrical contact with the region and extending to the rear portion of the pixel unit.
According to a further feature of the present invention, the regions are arranged in a two dimensional array, one of the dimensions corresponding to two of the regions.
According to a further feature of the present invention, the layered structure of each of the pixels further includes: (a) an intermediate transparent conductor and a second active layer containing liquid-crystal-type molecules, the intermediate transparent conductor and the second active layer being located between the first-mentioned active layer and the rear conductive layer, and (b) a third electrical connector in electrical contact with the intermediate transparent conductor and extending to the rear portion of the pixel unit.
There is also provided according to a further feature of the present invention, a front-lit color display panel comprising a plurality of pixels units, each pixel unit having a layered structure including a sequence of: a transparent conductor, an active layer containing stabilized chiral nematic liquid crystal, and a rear conductive layer, and wherein the plurality of pixel units includes a first group of pixel units in which the stabilized chiral nematic liquid crystal assumes a reflective state in which a first color is reflected, a second group of pixel units in, which the stabilized chiral nematic liquid crystal assumes a reflective state in which a second color is reflected, and a third group of pixel units in which the stabilized chiral nematic liquid crystal assumes a reflective state in which a third color is reflected.
According to a filer feature of the present invention, the rear conductive layer is transparent, and wherein the rear portion of the pixel unit includes black backing material.
According to a further feature of the present invention, one of the transparent conductor and the rear conductive layer is subdivided into a plurality of discontinuous regions, each of the regions being provided with a separate electrical connector in electrical contact with the region and extending to the rear portion of the pixel unit.
According to a Firer feature of the present invention, the pixel units are arranged such that each of the regions of a pixel unit of the first group is adjacent to one of the regions of a pixel unit from each of the second and third groups.
There is also provided according to a further feature of the present invention, a front-lit color display panel comprising a plurality of pixels units, each pixel unit having a layered structure including a sequence of: a filter, a transparent conductor, an active layer containing liquid-crystal-type molecules, and a rear conductive layer, and wherein the plurality of pixel units includes a first group of pixel units in which the filter is selectively transparent to a first color, a second group of pixel units in which the filter is selectively transparent to a second color, and a third group of pixel units in which the filter is selectively transparent to a third color.
There is also provided according to a further feature of the present invention, a front-lit color display panel comprising a plurality of pixels units, each pixel unit having a layered structure including a sequence of: a transparent conductor, an active layer containing liquid-crystal-type molecules and a dye, and a rear conductive layer, and wherein the plurality of pixel units includes a first group of pixel units in which the dye produces a reflective state in which a first color is reflected, a second group of pixel units in which the dye produces a reflective state in which a second color is reflected, and a third group of pixel units in which the dye produces a reflective state in which a third color is reflected.
There is also provided according to a further feature of the present invention, a front-lit display comprising a layered structure including a sequence of: a transparent conductor, a first active layer containing liquid-crystal-type molecules, an intermediate transparent conductor, a second active layer containing liquid-crystal-type molecules, and a rear conductor assembly including a mirror element.
According to a further feature of the present invention, the second active layer assumes a substantially black scattering state.